It is known that polyester fiber materials such as polyester fiber fabrics, polyester fiber cords and polyester fiber threads, have an excellent tensile strength and dimensional stability, and therefore, are useful as a reinforcing material for car tires, conveyer belts, V-belts and hoses. However, since the polyester fiber materials inherently have a poor bonding property to rubber, in order to utilize the polyester fiber material as a reinforcing material for rubber articles, it is required to significantly improve the bonding property of the polyester fiber material to rubber. For this reason, a number of approaches have been taken in attempts to improve the bonding property of the polyester fiber material to rubber. However, during the approaches, it was found that the improvement in the bonding property of the polyester fiber material to rubber causes the resultant improved polyester fiber material to have an excessively high stiffness, a poor processability in the shaping or molding process and a poor resistance to fatigue fracture. For example, U.S. Pat. No. 3,307,966 discloses a process for improving the bonding property of the polyester fiber material to rubber, by first impregnating the polyester fiber material with a first treating liquid containing a polyepoxide compound and an aromatic polyisocyanate compound, and then, by second impregnating the first impregnated polyester fiber material with a second treating liquid containing a reaction product of resorcin with formaldehyde and a rubber latex. The resultant product of the above-mentioned process exhibits a relatively superior bonding property to rubber. However, this known process causes the resultant product to have a relatively high stiffness and, therefore, it is difficult to bend the product during the shaping operation, and the product has a remarkably decreased resistance to fatigue fracture.
Japanese Patent Application Publication (Kokoku) No. 42-9004 discloses a process in which a polyester fiber material is treated with a first treating liquid containing an epoxy resin, a ethyleneimine compound and a rubber latex and, thereafter, the thus treated polyester fiber material is further treated with a second treating liquid containing a reaction product of resorcin with formal dehyde and a rubber latex. Also, U.S. Pat. No. 3,460,973 discloses a process in which a polyester fiber material is treated with a first treating liquid containing a lactam-blocked polyisocyanate compound, an emulsifying agent and a rubber latex, and then, with a second treating liquid containing a reaction product of resorcin with formaldehyde and a rubber latex. Furthermore, British Pat. No. 1,056,798 discloses a process in which a polyester fiber material is treated with a single treating liquid containing a blocked isocyanate compound, an epoxy resin and a rubber latex without using an additional treating liquid containing a resorcin-formaldehyde resin and a rubber latex. The above-mentioned three processes cause the resultant products to have a proper softness. However, these resultant products have a relatively low bonding property to rubber and, therefore, a poor rubber coverage. The term "rubber coverage" used herein refers to a percentage of total area of portions of the reinforcing material covered with rubber when the reinforcing material is peeled off from the rubber article in which the reinforcing material is embedded with in a rubber matrix.
As is clear from the above description, the conventional processes all failed to satisfy all of the requirements of the proper softness, high bonding property to rubber and high resistance to fatigue fracture of the rubber reinforcing polyester fiber materials.